REFEREED RESEARCH
Subirrigation for production of
native plants in nurseries —
concepts, current knowledge,
and implementation
Justin L Schmal, R Kasten Dumroese, Anthony S Davis,
Jeremiah R Pinto, and Douglass F Jacobs
ABSTRACT
Subirrigation, a method whereby water is allowed to move upward into the growing
medium by capillary action, has been the focus of recent research in forest and conservation nurseries growing a wide variety of native plants. Subirrigation reduces the
amount of water needed for producing high-quality plants, discharged wastewater,
and leaching of nutrients compared with traditional overhead irrigation systems. Recent research has shown additional benefits of subirrigation, such as enhanced crop
uniformity and improved outplanting performance. With these advantages and successful operational use in some locales, it seems likely that subirrigation would be of
use to a greater number of native plant nurseries. In this article, we provide an
overview of ebb-and-flow subirrigation technologies including potential benefits,
summarize the current state of research knowledge for native plant production, present special considerations for these systems, and offer a basic framework on how
growers can implement such a system.
Schmal JL, Dumroese RK, Davis AS, Pinto JR, Jacobs DF. 2011. Subirrigation for production of
native plants in nurseries — concepts, current knowledge, and implementation. Native Plants
Journal 12(2):81–93.
KEY WORDS
controlled-release fertilization, electrical conductivity, fertilizer-use efficiency, irrigation, nitrogen-use efficiency, water-use efficiency
N O M E N C L AT U R E
Plants: USDA NRCS (2011)
Insects: ITIS (2011)
Fungi: IFP (2011)
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N
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ursery managers, striving to produce quality seedlings, face government regulations on water use
(Oka 1993) and wastewater discharge (Grey 1991),
and mounting public concern about environmental contamination (Neal 1989). One area in which managers can simultaneously address all these concerns is through irrigation water
management. Use of subirrigation has shown promise in overcoming these challenges without compromising crop quality
and may concurrently provide other benefits (for example,
Dumroese and others 2006, 2007, 2011; Landis and others
2006; Bumgarner and others 2008; Davis and others 2008;
Pinto and others 2008). Specifically, growers can reduce the
quantity of water used during crop production, the amount of
water discharged from irrigation, and the fertilizers and chemicals present in discharged water.
Overhead irrigation systems can cover large areas, prevent
fertilizer salt accumulation in medium (Argo and Biernbaum
1995), and be installed with relatively little expense (Landis and
Wilkinson 2004). From a water and nutrient management perspective, however, these systems can be inefficient, may result
in significant fertilizer leaching, and are difficult to use on
small areas containing diverse species and (or) stocktypes. For
example, Dumroese and others (1995) found between 49 and
72% of water and 32 and 60% of nitrogen (N) applied using an
overhead irrigation system were discharged from a container
nursery. Additionally, a study examining nutrient uptake efficiency and leaching fractions in western white pine (Pinus
monticola Douglas ex D. Don [Pinaceae]) culture found that
irrigation water was leached at a rate of 1.3 l/m2 per d with N
losses of 8 mg N/m2 per d from leachate (Dumroese and others
2005). In another conifer seedling container study, Juntunen
and others (2002) recovered 11 to 19% of applied N and 16 to
64% of phosphorus in collected leachate. These examples of
discharged nutrients within wastewater demonstrate the impacts to the environment and the nursery budget. Culturally,
overhead irrigation also results in water interception and deflection (that is, “umbrella effect”) by the leaves of broadleaved plants (Figure 1). This effect may lead to uneven water
distribution (Landis and Wilkinson 2004), crop variability,
mortality in dry cavities (Dumroese and others 2006), and reduced irrigation application efficiencies (Beeson and Knox
1991). Yet despite these potential disadvantages, overhead irrigation systems are still standard practice in forest and conservation nurseries (Landis and others 1989a; Leskovar 1998).
The benefits of subirrigation have been demonstrated for a
variety of species and nursery systems (Dumroese and others
2006, 2007, 2011; Landis and others 2006; Bumgarner and others 2008; Davis and others 2008; Pinto and others 2008). Recent work with northern red oak (Quercus rubra L. [Fagaceae]), koa (Acacia koa A. Gray [Fabaceae]), pale purple
coneflower (Echinacea pallida (Nutt.) Nutt. [Asteraceae]),
‘ōhi‘a (Metrosideros polymorpha Gaudich. [Myrtaceae]), and
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Figure 1. Northern red oak, with its large leaves that form an
umbrella and prevent efficient overhead irrigation, can be readily
grown with subirrigation. Photo by Anthony S Davis; reprinted
from Dumroese and others 2007
blue spruce (Picea pungens Engelm. [Pinaceae]) demonstrates
the benefits and versatility of subirrigation and increased interest in this system (Dumroese and others 2007). Such benefits may include less water use, reduced labor inputs, improved
fertilizer efficiency, decreased liverwort and moss growth, plant
quality improvements, and more uniform crop growth. Incorporating subirrigation systems into nurseries can be easy and
low cost. Either commercially available equipment can be purchased to fit onto existing nursery benches, or custom, lowtech equipment can be constructed in-house to accommodate
a multitude of specialized needs (for example, Schmal and others 2007). Although several types of subirrigation systems are
available (for example, wick, trough, and flooded floor), we will
focus our discussion on ebb-and-flow subirrigation.
Although subirrigation systems are often used by the horticultural industry, this system, and its application in native
plant nursery production, is relatively new. Our intent is to
present an overview of the benefits of ebb-and-flow subirrigation as reported by recent research, examine special considerations with these systems, and provide practical information
about commercial and custom systems.
T H E S U B I R R I G AT I O N S Y S T E M
In subirrigation, a water-containing structure is flooded (Figure 2) until the water level contacts the medium (Figure 3).
Once contact is made, capillary action (the attraction of water
molecules for one another and other surfaces) moves water up
through the medium and throughout the container (Figure 4A)
S U B I R R I G AT I O N F O R N AT I V E P L A N T P R O D U C T I O N
Figure 2. Flooding of
subirrigation trays at
Hawai‘i Division of
Forestry and Wildlife
Kamuela (Waimea) State
Tree Nursery, on the
Island of Hawai‘i. Photo
by Douglass F Jacobs
Figure 3. Schematic of a typical ebb-andflow subirrigation system. An electronic
timer activates a submersible pump that
pushes water up into the subirrigation tray.
When the tray is full, the timer deactivates
the pump and the water drains back into
the reservoir tank. Illustration by Jim Marin
Graphics; reprinted from Dumroese and
others 2007
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JUSTIN L SCHMAL AND OTHERS
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B
A
Figure 4. Subirrigation works because water is drawn upward into the containers by capillary action of water (A). The amount
and speed of water uptake will depend on the porosity of the growing medium — the smaller the pores, the more that will be
absorbed (B). Illustration by Jim Marin Graphics; reprinted from Landis and Wilkinson 2004
84
(Landis and Wilkinson 2004). Growing medium pore space
and medium type (for example, Sphagnum peat) are the primary factors dictating saturation height and speed (Figure 4B)
(Landis and Wilkinson 2004). For a very well-drained medium, it may be that capillary action alone is insufficient to
maintain moisture levels at the medium surface during germination; therefore, subirrigation may need to be supplemented
with overhead irrigation to keep seeds and medium adequately
moist (Dumroese and others 2007). As with any new irrigation
system, it is essential to test and troubleshoot a subirrigation
system before going fully operational because of variability in
media, equipment, container types, and the moisture demands
of different species.
Several types of subirrigation systems are available on the
market (Landis and Wilkinson 2004); a closed system (Dumroese and others 2007) is one of the most promising systems for
forest and conservation nurseries. In a closed system, water is
pumped from a reservoir tank into a subirrigation tray (Figure
2); when the irrigation cycle is completed, the water returns to
the reservoir tank (Figure 3). Typically, water is held in the tray
until the medium is brought to field capacity; however, a range
(that is, low to high cost) of equipment is available to facilitate
and fine-tune this process. Many growers use automated
pumps with programmed flood cycles in which the pump fills
the tray, turns off for several minutes to allow the tray to completely drain, and repeats the process until the medium is
brought to field capacity. Alternatively, systems utilizing solenoid valves, which close the irrigation line for a specified dura-
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tion, can have a separate drain component where water is released after a certain amount of time. The USDA Forest Service,
Missoula Technology and Development Center has demonstrated the effective use of remote soil moisture probes for determining when to water bareroot nursery beds (Davies and Etter 2009). Integrating this technology into subirrigation systems
would further decrease water usage, labor inputs, and prevent
crop overwatering. Minimal maintenance is needed throughout the growing season to keep a subirrigation system working
properly; however, water losses through evaporation and crop
transpiration require periodic refilling of reservoir tanks.
SEEDLING QUALITY AND
MORPHOLOGY
Subirrigated plants are morphologically similar or superior to
those receiving overhead irrigation (Figure 5) (Coggeshall and
Van Sambeek 2002; Dumroese and others 2006, 2007, 2011;
Landis and others 2006; Bumgarner and others 2008; Davis
and others 2008). Pinto and others (2008) used subirrigation
to propagate pale purple coneflower seedlings that exhibited
better nutrition (that is, 11% greater N content per seedling),
13% greater nitrogen-use efficiency (NUE), less mortality, and
greater growth (15% taller and 14% more total dry weight)
than overhead irrigated seedlings receiving the same nutrient
rates (Figure 6). Subirrigated northern red oak seedlings had
increased aboveground biomass production and greater root
and shoot N contents during nursery culture when compared
S U B I R R I G AT I O N F O R N AT I V E P L A N T P R O D U C T I O N
with overhead irrigated seedlings (Figure 7) (Bumgarner and
others 2008).
Greater crop uniformity and outplanting performance have
also been noted with subirrigated seedlings. Bumgarner and
others (2008) showed that subirrigated northern red oak
seedlings had greater stem diameter growth following outplanting compared with overhead irrigated seedlings. Increased stem diameter was also noted in an outplanting trial of
koa seedlings (Davis and others 2011). Although not quantified, Landis and others (2006) noted that stem heights and diameters were very uniform in seedlings propagated using
subirrigation. Crop uniformity is easily attainable with subirrigation systems because the “umbrella effect” is eliminated
and an equal amount of water and nutrients are supplied to
each container. These results affirm that a correctly used subirrigation system yields similar or better seedling morphology,
quality, and outplanting success than does overhead irrigation.
R E D U C E D WAT E R U S E
Closed subirrigation systems allow for increased water-use efficiency because the only losses from the system are through
transpiration and evaporation. A study in Hawai‘i with ‘ōhi‘a
reported a 56% reduction in irrigation water using a subirrigation system; application values per container were 36 ml of water per d using fixed overhead irrigation and 16 ml/d with subirrigation (Dumroese and others 2006). The same study illustrated
the inefficiency of overhead systems because only 17% of the
applied water from the fixed overhead system was “used” by the
crop, as nearly 70% was errant spray and 13% of the applied water leached from the pots. Moreover, this study was conducted
at a remote site with very limited management; subirrigated
seedlings were probably watered more than necessary, indicating that additional water savings could have been made. Similarly, the Tamarac Nursery in Ontario, Canada, experienced a
70% savings in water and fertilizer use (Landis and Wilkinson
2004). These reductions in water use are attributable to the absence of lost leachate in closed subirrigation systems, the lack
of errant spray, and the elimination of the “umbrella effect” and
its subsequent need for extended irrigation periods to make up
for nonuniform irrigation coverage.
Figure 5. Blue spruce seedlings were slightly larger with subirrigation
compared to sprinkler irrigation in all 3 container types in height (A)
and diameter (B). Reprinted from Landis and others 2006
IMPROVED FERTILIZER-USE
EFFICIENCY
Most research with subirrigation in forest and conservation
nurseries has used controlled-release fertilizer (CRF). Incorporating CRF into the medium of nursery crops has many benefits
including improved fertilizer-use efficiency, less fertilizer pollution in discharge water, and the elimination of a need to rinse
foliage (Landis and Dumroese 2009). Combining the use of
JUSTIN L SCHMAL AND OTHERS
Figure 6. Mortality of pale purple coneflower seedlings grown with
subirrigation and fixed overhead irrigation. Reprinted from
Dumroese and others 2007
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5
A
100
a
4
80
Leaf
E
a
b
b
3
N
P
K
60
2
a
40
b
1
20
0
a
a
0
2.0
B
a
F
25
Stem
0.5
0.0
C
8
Root
Dry mass (g per component)
1.0
a
6
a
4
2
20
15
b
a
b
10
a
a
5
0
G
80
60
a
b
a
40
a
20
0
a
a
0
D
14
200
H
a
a
12
150
10
Plant
Nutrient content (mg per component)
a
b
1.5
a
8
b
100
a
6
b
4
50
2
0
a
a
Overhead
Subirrigated
0
Overhead
Subirrigated
Irrigation regime
Figure 7. Effects of irrigation method on northern red oak component dry weight (A–D) and nutrient content
(E–H) sampled 4 mo after sowing under controlled greenhouse environments. Treatments marked with
different letters are statistically different according to Tukey’s honesty significant difference test at α = 0.05.
Reprinted from Bumgarner and others 2008
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S U B I R R I G AT I O N F O R N AT I V E P L A N T P R O D U C T I O N
content (Figure 7E–H and 8) (Bumgarner and others 2008;
Pinto and others 2008; Dumroese and others 2011).
DECREASED PRESENCE OF
DAMAGING AGENTS
Figure 8. Foliar nitrogen concentrations in subirrigated koa seedlings
were higher than in overhead irrigated seedlings; concentrations
continued to rise with increasing rates of fertilization. Reprinted from
Dumroese and others 2011
CRF with a subirrigation system further enhances these benefits. To illustrate, the aforementioned study with ‘ōhi‘a (Dumroese and others 2006) used CRF in both overhead irrigated and
subirrigated treatments. The average concentration of N in
leachate from overhead irrigation was 43 ppm (24 g [0.85 oz] N
per replicate; equivalent to 5 g [0.18 oz] N leached per m2), representing a 3% loss of the total applied, which is very low when
compared with 32 to 60% losses with standard fertigation systems (Dumroese and others 1992, 1995). In spite of this, the average N concentration in subirrigation reservoir tanks was even
lower at 5 ppm (0.7 g [0.025 oz] N per replicate tank) (Dumroese and others 2006). Thus, a subirrigation system allows
nursery growers to save money by using less fertilizer.
Another added benefit of subirrigation is the persistence of
residual fertilizer salts in the medium and holding tanks
(Dumroese and others 2006, 2011). For example, after 9 mo of
irrigation using a 6-mo release CRF, electrical conductivity
(EC) in the medium of subirrigated seedlings was higher than
that of overhead irrigated plants (Dumroese and others 2006).
These residual fertilizer salts improve plant nutrient availability and fertilizer-use efficiency, while also serving as a source
of nutrient reserves during field establishment (Dumroese and
others 2006; 2011). The capture of leachate in closed subirrigation systems allows for recycling of nutrients that would otherwise be lost with overhead systems, leading to subsequent
improvements in nutrient-use efficiency and seedling nutrient
JUSTIN L SCHMAL AND OTHERS
Given the constant recycling of irrigation water in subirrigation
systems, the potential proliferation of disease is of concern and
has prevented some growers from experimenting with and (or)
using subirrigation systems. This concern is justified, as water
mold fungi, such as Phytophthora Bary (Pythiaceae) and Pythium Pringsh. (Pythiaceae), have been shown to spread in various subirrigation systems, including the ebb-and-flow system
used in floriculture and ornamental horticulture (Sanogo and
Moorman 1993; van der Gaag and others 2001; Oha and Son
2008), especially when surface water sources are used (Hong
and Moorman 2005). Whether or not disease ensues depends
on the plant and the pathogen (van der Gaag and others 2001),
and in several experiments, placement of diseased plants within
subirrigation areas spread less disease than when inocula were
added directly to the subirrigation reservoir (Sanogo and Moorman 1993; van der Gaag and others 2001).
We have yet to observe any waterborne disease issues with
subirrigation systems, probably because the growing media is
allowed to dry between irrigations; in conifer nurseries in the
Pacific Northwest, Phytophthora and Pythium are generally
only a problem when soil or media are persistently excessively
wet (Dumroese and James 2003). Moreover, the absence of disease in subirrigated seedlings may reflect plants that are healthier than those propagated by means of overhead irrigation.
When disease problems do arise, the cause(s) will likely result
from a failure to use clean propagules, disease-free medium,
and (or) disease-free water; not the subirrigation system itself.
Recent reviews discuss a myriad of cultural, physical, and
chemical methods that can be used in subirrigation systems to
control waterborne diseases (Newman 2004; Stewart-Wade
2011); control may be as simple as adding a surfactant to the
water (Stanghellini and others 2000).
Subirrigation may actually reduce some nursery pests. In an
‘ōhi‘a crop, subirrigation decreased moss and liverwort cover
on the medium to one-third that observed with overhead
TABLE 1
Average coverage of moss in each container with either fixed
overhead or subirrigation and percentage of those containers
with sporangium present (Dumroese and others 2006).
Irrigation
Moss coverage (%)
Sporangium (%)
Fixed overhead irrigation
50
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Subirrigation
15
23
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A
B
Figure 9. General lack of moss and liverwort growing on the surface of the medium of 6-mo-old subirrigated plants (A) versus that growing
with fixed overhead irrigation (B). Photos by R Kasten Dumroese; reprinted from Dumroese and others 2007
88
irrigation (Dumroese and others 2006) (Table 1; Figure 9).
Additionally, the moss growing in the fixed overhead containers was more mature (that is, had roughly four times more sporangium) than the moss in the subirrigation containers (Dumroese and others 2006) (Table 1; Figure 9). Because moss and
liverworts compete with the crop for fertilizer and light, they
can easily choke out small seedlings, reduce seedling growth,
or foster other pests, such as fungus gnats (Bradysia species
[Diptera: Sciaridae]).
The moistening of root crowns and foliage by overhead
irrigation can encourage the growth of foliar diseases such as
grey mold (Botrytis cinerea Pers. [Sclerotiniaceae]) (Landis and
others 1989b). Because subirrigation keeps foliage dry, it
lessens the potential for foliar diseases. Dumroese and others
(2006) did observe that subirrigating too frequently (that is,
overwatering) can lead to the proliferation of fungus gnat populations. Therefore, as previously mentioned, it is important to
test, monitor, and fine-tune subirrigation systems to obtain
optimal crop growth.
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S P E C I A L C O N S I D E R AT I O N S
Subirrigation tends to result in elevated EC values in the upper
regions of container medium compared to overhead irrigation
(Figure 10) (that is, EC values are high toward the top of containers and low toward the bottom) (Dumroese and others
2006, 2011; Bumgarner and others 2008; Davis and others
2008; Pinto and others 2008). The high EC results from water
evaporation at the medium surface that leaves behind soluble
salts (Landis 1988). These salts may be dissolved fertilizer from
the CRF or dissolved salts inherent in the water. Nurseries having irrigation water with high dissolved salts should be cautious with subirrigation due to the potential for elevated EC
(Landis and Wilkinson 2004). Although medium EC values
may be higher in subirrigation systems, research has shown
that these elevated values have not been detrimental to crops
(Dumroese and others 2006, 2011; Bumgarner and others
2008; Davis and others 2008; Pinto and others 2008). Because
root architecture is strongly influenced by EC (Jacobs and
S U B I R R I G AT I O N F O R N AT I V E P L A N T P R O D U C T I O N
For example, one subirrigation study conducted with blue
spruce noted that roots were growing between the bottom of
some containers and the surface of the subirrigation tray
(Dumroese and others 2007). Several solutions to remedy the
lack of root pruning with block-type containers exist and include using a copper coating within containers (for example,
Copperblock containers), or applying copper to the tray (for
example, Spin-Out), or covering the trays with copper mesh or
copper impregnated fabrics. Another quick and inexpensive
remedy for all container types is to use spacers to elevate the
containers so that the subirrigation tray is able to dry out completely between irrigations.
PERTINENT RESEARCH AREAS
Figure 10. Electrical conductivity values for overhead and subirrigated
pale purple coneflower seedling medium at three depths (3, 7, or 11
cm [1.2, 2.3, or 4.3 in]) from the top of the container. Measurements
were taken 93 d after sowing. Reprinted from Pinto and others 2008
others 2003) and the upper layer of medium typically has the
least amount of root dry mass (Todd and Reed 1998; Pinto and
others 2008), the negative impacts from elevated upper layer
EC are diminished.
Although EC thresholds for several conifer (Landis 1988;
Jacobs and Timmer 2005) and herbaceous species (Scoggins
2005) exist, little information is available for many hardwood
and other native plant species. Extracted medium solutions
from subirrigated containers of northern red oak had EC values above recommended thresholds (Jacobs and Timmer
2005), but these levels did not negatively impact seedling
growth (Bumgarner and others 2008). Medium EC at different
depths can quickly be assessed with an electrical conductivity
meter, such as a Field ScoutTM (Spectrum Technologies Inc,
Plainfield, Illinois). Because different measurement techniques
can yield different values, we recommend that growers continually monitor crops and look at the overall trend in EC and
subsequent plant response. Vigilant growers can then establish
EC threshold values, based on their monitoring equipment and
specific to their crops, and thereby avoid salt damage. If
medium EC values exceed thresholds, an overhead application
of clear irrigation water can immediately and drastically lower
medium EC. For example, Davis and others (2008) had EC values in subirrigated containers that averaged 3.31 dS/m at a
depth of 1 cm (0.39 in). Following overhead irrigation with
clear water, average EC values dropped to 0.77 to 1.53 dS/m.
Observations during several research studies indicate that a
lack of air space between the bottom of the containers and the
subirrigation tray prevents seedling roots from “air pruning.”
JUSTIN L SCHMAL AND OTHERS
Although recent studies have provided much beneficial information about the effects of subirrigation on the culture of plants
in forest tree seedling and conservation nurseries, several pertinent areas still require investigation. First, while fertigation (fertilizer diluted with the irrigation water) is extensively used with
overhead irrigation systems, we need more information about
combining fertigation and subirrigation specific to forest tree
seedling and conservation nurseries. Research in this area with
horticultural species has shown the applicability of fertigation
to subirrigation systems (Treder and others 1999; James and
van Iersel 2001a, b), potentially allowing reduction of fertilizer
application rates (Zheng and others 2005). Second, we need
more information about the potential movement of waterborne
diseases in subirrigation systems. Quantitative studies that
compare disease spread, persistence, severity (for example,
through inoculation of the crop with pathogens and spores),
and possible remedies, such as biological controls and welldrained media, are warranted in forest tree seedling and conservation nursery subirrigated systems. Such studies may help
eliminate a “fear-of-the-unknown” that may prevent some
growers from realizing the benefits of subirrigation. Third, we
need more information about the suitability of subirrigation for
different plant types (for example, grasses, forbs, shrubs, trees)
as well as the diverse number of species in those types; therefore, more trials with more species are necessary.
IMPLEMENTING A
S U B I R R I G AT I O N S Y S T E M
Subirrigation systems can be quickly installed, used for a variety
of container types, and accommodate a wide range of production scales. The abundance of manufacturers offering subirrigation products combined with the inherent simplicity of basic
subirrigation systems allow for large ranges in the complexity
and price of these systems. The cost of some commercial systems may preclude their use in nurseries with budget constraints. Additionally, nursery growers may not be able to
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Figure 11. Schematic of the subirrigation
system used at USDA Forest Service Lucky
Peak Nursery. Reprinted from Schmal and
others 2007
90
justify high up-front costs of a new system if they are initially
skeptical of the cost versus benefits. Fortunately, materials to
construct subirrigation systems in-house are readily available,
and constructing custom systems can be a quick, cost-effective,
and efficient way to subirrigate a variety of seedlings.
At the USDA Forest Service Lucky Peak Nursery, children’s
wading pools were used as part of a subirrigation system to water a variety of seedlings in 3.8-l (1-gal) Tree Pots (Schmal and
others 2007). The system consisted of a main PVC irrigation
pipe, a Rain Bird-type irrigation timer, and PVC distributor
pipes with individual hand valves running to each pool (Figure
11). The pools were supported above the ground by metal
cooler shelving placed on cinder blocks. About 130 containers
fit into each pool (Figure 12), and plants were being grown in
this system for 1 to 2 y. Approximate cost on a per pool basis
was $20 USD, not including the supporting structures. Other
low-cost options for in-house constructed units include flood
floor systems that consist of a raised perimeter of concrete
blocks and lumber covered with pond liner (Landis and
Wilkinson 2004).
Commercial subirrigation systems fall into 2 general categories: bench systems and flood floor systems. Both systems
are highly customizable, accommodate practically any type and
size of container, and can be configured to suit a variety of
space and budget constraints. With bench systems, subirrigation trays, troughs, or bench liners are placed on top of nursery
benches, and crop containers are subsequently placed within
these structures. The benches must be sturdy enough to support the weight of the subirrigation water, the trays, and the
N AT I V E P L A N T S | 1 2 | 2 | S U M M E R 2 0 1 1
saturated containers. In addition to structural support, precise
leveling is required to completely drain the water following an
irrigation event; complete drainage facilitates air pruning and
minimizes evaporative losses.
With flood floor systems, subirrigation water is contained
by concrete or other impermeable material, and crop containers sit directly on the floor surface, leaving only as much room
below as needed for air pruning. For many of these commercial
systems, concrete must be poured and precisely leveled and
enclosed plumbing routed, resulting in significant labor and
Figure 12. Top view of subirrigation using a children’s wading pool
filled with approximately 130 3.8-l (1-gal) Tree PotsTM at the USDA
Forest Service Lucky Peak Nursery. Photo by Justin Schmal; reprinted
from Schmal and others 2007
S U B I R R I G AT I O N F O R N AT I V E P L A N T P R O D U C T I O N
material costs. These systems may be a better option for nurseries planning to expand growing areas, rather than for those
that already have an ample supply of suitable bench space, because many standard benches can be modified to accommodate subirrigation trays. Some advantages of flood floor systems are their inherent durability, allowance of unobstructed
traffic, and the ability to combine floor heating.
In addition to the items mentioned above, most subirrigation systems require a few other elements. In a closed system
where water will be stored and recycled, a reservoir tank is necessary. As a general rule, 3.78-l (1-gal) will flood 0.09 m2 (1 ft2)
to a depth of 4.1 cm (1.6 in). Livestock water tanks are a good,
inexpensive option for subirrigation systems and are available
at many local farm supply stores in a range of volumes. Because of the propensity for algal growth, it is important to use
opaque materials that inhibit light penetration for all subirrigation components. Level (float) switches can be used to trigger the automatic “topping-off” of storage tanks when evaporation and transpiration have lowered water levels. Another
essential item is a pump (for example, sump pump without a
check valve) suitably sized for the subirrigation system’s volume and configuration. Manufacturers offering complete subirrigation systems can provide recommendations on the sizes
of pumps needed for custom systems. For highly automated
systems, a controller can be used in conjunction with solenoid
valves to water different zones at different times. Below is a listing of several companies offering a wide range of commercial
subirrigation products and systems.
Beaver Plastics Ltd carries FlowTraysTM for holding small
numbers of containers and FlowBenchTM sheets that come
in a range of widths and can be joined together to accommodate any length bench. These products are constructed from
sunlight-resistant ABS plastic.
Beaver Plastics Ltd
7-26318-TWP RD 531A
Acheson, AB T7X 5A3 Canada
TEL: 888.453.5961
FAX: 780.962.4640
E-MAIL: growerinfo@beaverplastics.com
WEBSITE: http://www.beaverplastics.com
Stuewe & Sons Inc offers a wide variety of containers and pots.
They also have smaller flow trays that would be well suited to
growers wanting to conduct subirrigation trials. Their largest
flow tray ($67 USD each) can hold 4 Ray Leach Cone-tainerTM
trays, while their smallest ($29.50 USD each) can hold a single
tray. These flow trays are made of sunlight-resistant, 5 mm
thick, black or white ABS plastic.
Stuewe & Sons Inc
31933 Rolland Drive
Tangent, OR 97389 USA
TEL: 800.553.5331
JUSTIN L SCHMAL AND OTHERS
FAX: 541.754.6617
E-MAIL: info@stuewe.com
WEBSITE: http://www.stuewe.com
Midwest GROmaster Inc offers Ebb-FloTM benches and trays
that are pre-cut, pre-drilled, and ready for assembly. These
custom-fit trays can be retrofitted to existing benches that are
easily leveled using leveling bars. Tray widths are available
from 45.7 cm (1 ft 6 in) to 1.98 m (6 ft 6 in) and range in price
from $6.92 USD/ft2 to $3.25 USD/ft2 (not including leveling
bars). The company also offers a variety of irrigation and fertigation controls.
Midwest GROmaster Inc
PO Box 1006
St Charles, IL 60174 USA
TEL: 847.888.3558
FAX: 630.443.5035
E-MAIL: sales@midgro.com
WEBSITE: http://www.midgro.com
Zwart Systems designs, sells, and installs custom flood floor
systems as well as flood trays. They also offer a full line of greenhouse and irrigation supplies, including water storage tanks.
Zwart Systems
4881 Union Road
Beamsville, ON L0R 1B4 Canada
TEL: 800.932.9811
FAX: 905.563.9238
E-MAIL: info@zwartsystems.ca
WEBSITE: http://www.zwartsystems.ca
TrueLeaf Technologies designs and sells custom flood floor
systems to fit any greenhouse or outdoor growing situation.
They also design and provide complete flood bench system
packages. According to the company’s website, cost of design,
materials, and labor is on the order of $4.50 to $6.00 USD/ft2.
TrueLeaf also sells water filtration and purification systems for
large-scale nursery use.
TrueLeaf Technologies
PO Box 750967
Petaluma, CA 94975 USA
TEL: 800.438.4328
FAX: 707.794.9663
E-MAIL: info@trueleaf.net
WEBSITE: http://www.trueleaf.net
Although the upfront costs of subirrigation systems may
appear high, nursery managers could quickly recapture the expense in the form of labor savings, reduced equipment maintenance, and improved plant quality. According to Midwest
GROmaster’s website, a customer with nearly 0.4 ha (1 ac) of
Ebb-FloTM benches was able to recapture their upfront costs
within 2 y from labor savings alone. When combining labor
savings with reductions in water and fertilizer usage, investing
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in a subirrigation system may prove a wise economical decision. Similarly, given the possible gains in crop quality, the
alleviation of nursery noncompliance with water usage restrictions during dry periods, and increasing water quality legislation make the implementation of a subirrigation system an intelligent and timely decision.
SUMMARY
Nursery managers require the most effective and efficient cultural practices available in order to produce uniform, highquality stock for their customers while addressing public concern about, and increasing government regulations on,
sustainability and environmental contamination. Subirrigating
crops provides several proven advantages over the standard
overhead irrigation systems used at most forest and conservation nurseries, specifically reduced water use, less labor inputs,
improved fertilizer efficiency, decreased liverwort and moss
growth, and greater crop uniformity. Subirrigation systems
range from simple low-tech applications to high-tech, automated ones. Innovative nursery managers can probably fabricate their own systems, or they can seek the expertise of
companies that offer a wide variety of quality subirrigation
products to customize and integrate subirrigation systems into
existing nursery infrastructures. Although not yet widely used
at forest and conservation nurseries, more trials and demonstrations, particularly by growers, will likely lead to discovery
of additional benefits and increase the use of subirrigation. Our
environment as well as our nursery crops stand to benefit from
increased use of subirrigation, because contamination is minimized and crop quality is enhanced by this system. With
worldwide expansions in forest certification, more forest industries going “green,” and increasing demand for native
plants for myriad reasons, nurseries using subirrigation can
market their efforts to minimize their environmental footprint
and to improve their sustainability.
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A U T H O R I N F O R M AT I O N
Justin L Schmal
Research Associate
Douglass F Jacobs
Professor of Forest Regeneration
djacobs@purdue.edu
Hardwood Tree Improvement and Regeneration Center
Department of Forestry and Natural Resources
Purdue University
West Lafayette, IN 47907-2061
R Kasten Dumroese
National Nursery Specialist
kdumroese@fs.fed.us
Jeremiah R Pinto
Research Plant Physiologist
jpinto@fs.fed.us
USDA Forest Service
Rocky Mountain Research Station
1221 South Main Street
Moscow, ID 83843-4211
Anthony S Davis
Assistant Professor of Native Plant Regeneration
and Silviculture
Director, Center for Forest Nursery and Seedling
Research
Department of Forest Ecology and Biogeosciences
University of Idaho, Moscow, ID 83843-4211
asdavis@uidaho.edu
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